1. Field of the Invention
The present invention relates to a control circuit and a method thereof, and particularly to a control circuit and a method thereof that can control voltage crossing a power switch of a switched-mode power converter.
2. Description of the Prior Art
A switched-mode power converter, also called a “switched-mode power supply” (SMPS), is an electronic power converter. The switched-mode power converter is used for converting and providing input power to a load. Generally speaking, a voltage level of the input power of the switched-mode power converter is different from a voltage level of output power of the switched-mode power converter. Compared to a linear power converter, the switched-mode power converter not only can provide higher power conversion efficiency, but can also have smaller area. In a transformer-coupled switched-mode power converter, the transformer-coupled switched-mode power converter utilizes a transformer to isolate a power input terminal from a power output terminal, where a side of the transformer near the power input terminal is called a primary side, and another side near the power output terminal is called a secondary side. The primary side of the transformer includes a switch for being controlled by pulse width modulation (PWM). The transformer-coupled switched-mode power converter is further divided into a forward switched-mode power converter and a flyback switched-mode power converter.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a flyback switched-mode power converter 100 according to the prior art. As shown in FIG. 1, the switched-mode power converter 100 mainly includes a rectifier 104, a transformer 106, a regulator 108, a photo coupler 110, a control circuit 112, and a power switch SW1. The switched-mode power converter 100 is coupled to an alternating current voltage source 102, and provides an approximate direct current voltage to the transformer 106 and the control circuit 112 by the rectifier 104. The control circuit 112 is used for providing a pulse width modulation signal VOUT to control the power switch SW1. The transformer 106 provides input power from a primary side to a secondary side by turning-on and turning-off of the power switch SW1. The regulator 108 and the photo coupler 110 are used for providing a feedback compensation signal VCOMP of an output voltage of the switched-mode power converter 100 to the control circuit 112. The control circuit 112 can execute pulse width modulation operations by comparing the feedback compensation signal VCOMP and current IP flowing through the power switch SW1.
However, the switched-mode power converter 100 in FIG. 1 has disadvantages as follows: when the switched-mode power converter 100 is activated, the output voltage of the secondary side starts to increase from a ground voltage level. A characteristic of the control circuit 112 can quickly increase speed of power conversion of the transformer 106. Therefore, the feedback compensation signal VCOMP is a high voltage which causes the power switch SW1 to have a higher switch frequency.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating a relationship between the feedback compensation signal VCOMP and the current Ip flowing through the power switch SW1 when the switched-mode power converter 100 is activated. As shown in FIG. 2, during a first switch period of the power switch SW1, a switch frequency of the power switch SW1 is higher, and time for turning-off of the power switch SW1 is shorter, resulting in the power switch SW1 entering a continuous mode during a second switch period of the power switch SW1. That is to say, the primary side of the transformer 106 can not discharge completely when the power switch SW1 enters a next switch period, so current Ip flowing through the power switch SW1 gradually becomes larger. Because a voltage crossing the power switch SW1 is almost proportional to the current IP, the higher current IP may result in the voltage crossing the power switch SW1 being so high as to damage itself.
Therefore, those skilled in the art need a method for controlling voltage crossing a power switch of a switched-mode power converter and a circuit thereof, and the method should effectively control the voltage crossing the power switch of the switched-mode power converter to prevent the power switch from being damaged when the switched-mode power converter is activated.